Investigation of Azithromycin Minimum Inhibitory Concentration Values and Carbapenem Resistance in Salmonella and Shigella Clinical Isolates

Increasing resistance to first-line antibiotics used in the treatment of infections caused by Salmonella and Shigella species is emerging. Azithromycin presents a good alternative treatment option for Salmonella and Shigella infections. However, there are limited data regarding the susceptibility of azithromycin in Turkey. In this study, we aimed to evaluate the susceptibility of Salmonella and Shigella species to azithromycin, to determine and compare the minimum inhibitory concentration (MIC) values and disk diffusion zone diameters. In addition, susceptibility to meropenem and first-line antibiotic options in isolates was also investigated. A total of 170 Salmonella, 76 Shigella clinical isolates collected between 2014 and 2018 in our hospital were tested for their susceptibility to azithromycin, meropenem, ampicillin, pefloxacin, trimetoprim-sulfamethoxazole, ceftazidime, and cefotaxime. Isolates were identified by matrix-assisted laser desorption ionization time of flight mass spectrometry. The isolates were confirmed and serotyped by the reference laboratory using the conventional slide agglutination method. Susceptibility of the isolates to azithromycin and other antibiotics was evaluated by Kirby-Bauer disk diffusion method. MIC values of azithromycin were determined by the reference broth microdilution method. Combined disk diffusion test was used for the detection of extended spectrum beta-lactamase (ESBL) production. Polymerase chain reaction was performed for macrolide and carbapenem resistance genes and the detected resistance genes were confirmed by sequencing. Of the 76 Shigella isolates tested, 64 (84.2%) were identified as Shigella sonnei, 10 (13.2%) as Shigella flexneri, one (1.3%) as Shigella boydii, and one (1.3%) as Shigella dysenteriae. Among the 170 Salmonella isolates, 131 (77%) were identified as Salmonella enteritidis, 11(6.5%) as Salmonella Typhimurium, 8 (4.7%) as Salmonella Kentucky, 5 (2.9%) as Salmonella Paratyphi B, 4 (2.4%) as Salmonella Infantis, 3 (1.8%) as Salmonella Cholerasuis, and 8 (4.7%) as other serovars (Salmonella Agona, Salmonella Dabou, Salmonella Gallinarum, Salmonella Hadar, Salmonella Muenchen, Salmonella Newport, Salmonella Paratyphi C, Salmonella Senftenberg), respectively. ESBL production was determined as 7.9% (6/76) in Shigella isolates and 2.9% (5/170) in Salmonella isolates. A carbapenem resistant S.Senftenberg isolate positive for the bla OXA-48 resistance gene was detected in our study. Meropenem MIC value of the isolate was detected as > 32 mu g/ml with gradient diffusion test. Among all isolates, only one S.boydii isolate was detected as resistant to azithromycin with a MIC value of 128 mu g/ml. The isolate was positive for the existence of mphA gene by PCR. In the disk diffusion test, azithromycin inhibition zone diameters were >= 12 mm in all of the tested isolates, except for the azithromycin-resistant isolate, and the azithromycin MICs were determined as <= 16 mu g/ml by broth microdilution. Increasing resistance to commonly used antibiotics in Salmonella and Shigella species is emerging. The detection of a carbapenem-resistant Salmonella isolate in our study indicates that the spread of carbapenem resistance to other Enterobacterales species may cause global problems. Antimicrobial susceptibility testing of azithromycin for Salmonella and Shigella species has been difficult to establish due to the lack of approval in vitro breakpoints for all species. Consequently, our data shows that azithromycin exhibits as a good alternative therapeutic choice for the treatment of gastrointestinal diseases caused by Salmonella and Shigella species. Further studies are needed to provide appropriate in vitro breakpoints supported by clinical data.